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Fair Value Measurements (Details 1) - Level 3 - USD ($) $ in Millions	9 Months Ended	12 Months Ended
	Sep. 30, 2017	Dec. 31, 2016
NUG contracts
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	$ 1	$ 1
Beginning Balance, Derivative Liabilities	(108)	(137)
Beginning Balance, Net	(107)	(136)
Unrealized gain (loss), Derivative Assets	0	2
Unrealized loss, Derivative Liabilities	(14)	(17)
Unrealized loss, Net	(14)	(15)
Purchases, Derivative Assets	0	0
Purchases, Derivative Liabilities	0	0
Purchases, Net	0	0
Settlements, Derivative Assets	(1)	(2)
Settlements, Derivative Liabilities	30	46
Settlements, Net	29	44
Ending Balance, Derivative Assets	0	1
Ending Balance, Derivative Liabilities	(92)	(108)
Ending Balance, Net	(92)	(107)
FTRs
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	7	8
Beginning Balance, Derivative Liabilities	(6)	(13)
Beginning Balance, Net	1	(5)
Unrealized gain (loss), Derivative Assets	0	(6)
Unrealized loss, Derivative Liabilities	(2)	(4)
Unrealized loss, Net	(2)	(10)
Purchases, Derivative Assets	5	16
Purchases, Derivative Liabilities	(2)	(7)
Purchases, Net	3	9
Settlements, Derivative Assets	(7)	(11)
Settlements, Derivative Liabilities	8	18
Settlements, Net	1	7
Ending Balance, Derivative Assets	5	7
Ending Balance, Derivative Liabilities	(2)	(6)
Ending Balance, Net	3	1
FTRs | FES
Reconciliation of changes in the fair value of FTRs contracts
Beginning Balance, Derivative Assets	4	5
Beginning Balance, Derivative Liabilities	(5)	(11)
Beginning Balance, Net	(1)	(6)
Unrealized gain (loss), Derivative Assets	0	(4)
Unrealized loss, Derivative Liabilities	(1)	(3)
Unrealized loss, Net	(1)	(7)
Purchases, Derivative Assets	1	10
Purchases, Derivative Liabilities	(1)	(5)
Purchases, Net	0	5
Settlements, Derivative Assets	(4)	(7)
Settlements, Derivative Liabilities	6	14
Settlements, Net	2	7
Ending Balance, Derivative Assets	1	4
Ending Balance, Derivative Liabilities	(1)	(5)
Ending Balance, Net	$ 0	$ (1)

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Amount of purchases of financial instrument classified as an asset measured using unobservable inputs that reflect the entity's own assumption about the assumptions market participants would use in pricing.

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-Publisher FASB
-Name Accounting Standards Codification
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